A helicopter main rotor assembly is subjected to various aerodynamic, inertial, and centrifugal forces and moments during flight operations and to main rotor blade static droop when the helicopter powerplant is shut down. The main rotor assembly is designed to accommodate such forces and moments through the structural and functional characteristics of the various structural elements comprising the main rotor assembly. Of particular concern in designing a helicopter main rotor assembly are centrifugal loading (due to rotation of the rotor blades), torsional loading (due to pilot pitch control inputs), flapwise loading (due to out-of-plane blade motions, i.e., flapping), and edgewise or chordwise loading (due to in-plane blade motions, i.e., lead or lag).
Many prior art main rotor assemblies (older helicopters or those at the lower end of the cost spectrum) utilize mechanical mechanisms to react centrifugal, pitch, flapping, and/or lead-lag loads developed in the main rotor assembly. Blade attachment bolts have been utilized to transfer centrifugal loads from the main rotor blades to the main rotor hub structure. Mechanical linkages have been utilized to input pilot pitch changes to the main rotor blades. Bearings in the form of rolling element or elastomeric bearings have been utilized in mechanical hinges to react the pitch, flapping and lead-lag motions experienced by the main rotor blades.
Offset flapping and lead-lag hinges have been incorporated in main rotor assemblies to react flapping and lead-lag loads, respectively, of the main rotor blades. While such hinges have generally functioned in a satisfactory manner, the structural configurations of such hinges increase the mechanical complexity and weight of the main rotor assembly due to the load carrying capability required of such hinges.
The increased reliability, adaptability, reproducibility, and flexibility available from composite structures, due to the advancements in composite materials and/or fabrication techniques, has led to the increased use of composite materials in helicopter main rotor assemblies. Individual composite structural elements may be designed and fabricated for main rotor assemblies to react a plurality of the main rotor assembly loading effects described hereinabove, thereby reducing the number of mechanical mechanisms required in the main rotor assembly to react centrifugal, pitch, flapwise, and/or chordwise loads and providing a concomitant reduction in the overall weight and complexity of the main rotor assembly. The elimination of the offset flapping and lead-lag hinges in main rotor assemblies through the use of composite structural members has resulted in "bearingless" main rotor assemblies. The composite structural members of a bearingless main rotor assembly may be described as "flexbeams" or "crossbeams" due to the structural and/or functional characteristics of such composite structural members.
An exemplary composite crossbeam of a bearingless main rotor assembly is described in U.S. Pat. No. 4,746,272. The integral composite crossbeam described in the '272 patent is designed and fabricated to segregate the flapwise shear strain from the torsional shear strain to improve the fatigue performance of the crossbeam. An inboard flexure portion of each leg of the integral crossbeam is comprised of unidirectional composite fibers overwound or wrapped with a +/-45.degree. composite wrap that forces torsional deflections outboard of the inboard flexure portion. The inboard flexure portion, therefore, reacts flapwise loads while an outboard flexure portion of each leg of the integral crossbeam reacts chordwise and torsion loads. The composite crossbeam provides an integral mechanical interface between opposed main rotor blades which accommodates centrifugal loading experienced by the opposed main rotor blades.
Representative examples of the use of composite structures in bearingless main rotor assemblies are illustrated in FIGS. 1, 2. FIG. 1 illustrates a bearingless main rotor assembly of the type utilized on MBB 108 helicopters wherein individual composite flexbeams are mechanically coupled to the main rotor assembly hub by vertical pins. Centrifugal loads of the main rotor blades are reacted by the bolted connections of the vertical pins. The composite flexbeams are designed and fabricated to react the flapwise and chordwise loads experienced by the main rotor blades as well as the torsion loads produced by the pitch control input mechanisms. Each flexbeam includes an outboard joint that provides a mechanical interconnection for the inboard end of a main rotor blade spar and the outboard end of a torque tube (one element of the pitch control input mechanism).
FIG. 2 illustrates a bearingless main rotor assembly of the type utilized on Bell 680 helicopters wherein integral composite crossbeams are incorporated as part of the hub configuration of the main rotor assembly. As with the flexbeams illustrated in FIG. 1, the integral composite crossbeams are designed and fabricated to react the flapwise and chordwise loads experienced by the main rotor blades as well as the torsion loads produced by the pitch control input mechanism.
Strategic and tactical considerations in the military utilization of helicopters has led to a requirement for helicopters having main rotor assemblies that may be readily reconfigured for rapid deployment, routine transport, and/or storage through reduction in the lateral/longitudinal structural envelope of the helicopter. Reconfiguration may be accomplished by main rotor blade removal or main rotor blade folding. While the bearingless main rotor assemblies illustrated in FIGS. 1, 2 may be reconfigured to reduce the structural envelope of the helicopter, each of the illustrated main rotor assembly configurations has disadvantages that makes them less than optimally suited for main rotor assembly reconfiguration.
The main rotor assembly depicted in FIG. 1 is representative of an inboard reconfiguration joint. To reconfigure such a main rotor assembly by the removal technique requires time and labor not only for the removal of the vertical hinge bolt connections, but also the disconnection of the pitch control input mechanism for each main rotor blade. Each flexbeam, torque tube, main rotor blade combination may then be detached from the hub structure to provide a helicopter having a significantly reduced structural envelope. Reconfiguring the helicopter for flight operations, however, is also a time consuming and labor intensive procedure. Not only must the flexbeam, torque tube, main rotor blade combinations and pitch control input mechanisms be reconnected, but the reassembled main rotor assembly requires a check-out flight before the resumption of normal flight operations.
The main rotor assembly of FIG. 1 may also be reconfigured by the folding technique by removing one of the connection bolts from each of the vertical hinges and folding the respective flexbeam, torque tube, main rotor blade combination about the remaining bolted connection. In addition to the flapwise loads that are reacted at the vertical hinge positions, the bolted hinge connections must also be designed to support the weight of the flexbeam, torque tube, main rotor blade combination during the folding operation, which typically necessitates a stronger (and heavier) vertical hinge configuration than would otherwise be required. Furthermore, the configuration of the main rotor assembly hub structure, i.e., the proximity of adjacent vertical hinges, presents clearance problems in folding adjacent flexbeam, torque tube, main rotor blade combinations.
Based upon the foregoing disclosure, it may be appreciated that a main rotor assembly having an inboard reconfiguration joint is not eminently suited for reconfiguration by either the blade removal or blade folding techniques.
The bearingless main rotor assembly depicted in FIG. 2 is representative of an outboard reconfiguration joint. While this type of main rotor assembly configuration may be reconfigured by either the removal or the folding technique, the spatial location of the outboard reconfiguration joint makes this type of bearingless main rotor assembly not particularly well-suited for either type of reconfiguration.
To accommodate the various main rotor blade loadings described hereinabove, in particular, the torsion loading from the pitch control input mechanisms, the integral composite crossbeams (as well as the integral composite flexbeams of FIG. 1) extend outwardly from the rotor hub structure a significant distance radially. This spatial location of the outboard reconfiguration joint makes access to the joint for the reconfiguration procedure exceedingly difficult. The outboard reconfiguration joints of the integral crossbeam are situated in the aerodynamic flowpath of the main rotor assembly (at about the 25% station of the rotor radius span) such that the shape of the reconfiguration joint and/or joint access panel causes an increase in induced drag. In addition, to accommodate reconfiguration by means of the folding technique, the outboard reconfiguration joint must be upscaled in size to provide sufficient structural strength for blade folding about one connection bolt. Such an outboard reconfiguration joint, even if aerodynamically configured, acts as a discontinuity in the aerodynamic flowpath, the net result being that the shroud causes an increase in induced aerodynamic drag for such a bearingless main rotor assembly.
A need exists for a helicopter main rotor assembly that includes reconfiguration joints that facilitate reconfiguration of the helicopter for rapid deployment, routine transport, and/or storage of the helicopter through reduction in the lateral/longitudinal structural envelope of the helicopter. The reconfiguration joint should be readily integrable as elements of the composite components comprising the main rotor assembly that are functional to react the centrifugal, flapwise, chordwise, and torsional loads of the main rotor assembly. Further, the reconfiguration joint should be readily accessible to maintenance personnel and should facilitate an optimized reconfiguration procedure in terms of time and labor.